1. Field of the Invention
The invention relates to a wheel condition determination apparatus that determines the condition of a wheel provided in a vehicle.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-11-240315 describes an example of an apparatus that determines the condition of a wheel provided in a vehicle. The apparatus includes a tire tag and a transponder. The tire tag, provided inside a tire, includes a sensor and a battery. The sensor detects a predetermined parameter indicating the wheel condition. The transponder, provided in the wheel rim, transmits the parameter detected by the sensor of the tire tag to the vehicle. Japanese Patent Application Publication No. JP-A-11-254926 also describes an apparatus that determines the condition of a wheel. The apparatus includes a unit, provided in the wheel rim, that includes a sensor, a battery, a microchip, and an antenna. The sensor detects a predetermined parameter indicating the wheel condition. The unit transmits the parameter detected by the sensor to the vehicle.
In the described apparatuses, the battery can supply electric power for a relatively long period. However, the amount of electric power supplied to the sensor and related components should be minimized to determine the wheel condition for a long time. Japanese Patent Application Publication No. JP-A-11-020427 describes an example of a tire air-pressure detection apparatus for a wheel. The tire air-pressure detection apparatus includes an air-pressure sensor, a transceiver, and a centrifugal-force switch that is turned on when centrifugal force generated by the rotation of a wheel becomes equal to or greater than a predetermined value. The air-pressure sensor detects the air pressure and the transceiver transmits and receives data only when the centrifugal-force switch is ON, that is, only when the vehicle is moving. This reduces the amount of electric power consumed to determine the wheel condition.
The information useful for improving the running performance and safety of the vehicle includes not only the aforementioned tire air-pressure but also other parameters. Therefore, a technology where a plurality of sensor units are provided in a wheel, and each sensor detects a different parameter indicating the wheel condition has been recently proposed. In this technology as well, the amount of electric power required for detection of the condition of the wheel and data communication should be minimized. However, if the centrifugal-force switch is provided in each of the plurality of sensor units as in the conventional tire air-pressure detection apparatus, the cost of the entire apparatus increases, and each sensor unit occupies a large space.